Methods and apparatus for distinguishing between bodies according to their translucency

ABSTRACT

Relatively opaque and relatively translucent bodies are distinguished from one another by illuminating a limited area of the surface of each body and determining the amount of light emitted from the surface of the body outside the limited area, the amount of such emitted light increasing with increasing translucency of the body. A coaxial light-transmitting and lightreceiving arrangement is preferred, using a common lens, and using adjacent fibre-optic transmitting and receiving surfaces. The invention has one application in distinguishing between ore fragments which comprise a translucent matrix (e.g., quartz or quartzite) and greater or lesser amounts respectively of an opaque mineral (e.g., uraninite) non-uniformly dispersed therein, and can be applied to physically separate the fragments according to their mineral content.

XR 3 s17 86 a 266 United State ,76 1111 3,786,266 Reid et al. 1 1451 Jan. 15, 1974 METHODS AND APPARATUS FOR 3,436,757 4/1969 Schwab 250/227 x DISTINGUISHING BETWEEN BODIES B k i m s ACCORDING TO THEIR TRANSLUCENCY 3,518,440 6/1970 Hanson et al...... 250/227 X [75] Inventors: Colin David Reid, Newbury, 3,496,363 2/1970 Rome 250/277 X England; Ian Daniel Ritchie, Belfast, Northern Ireland Primary Examiner-Walter Stolwein [73] Assignee: United Kingdom Atomic Energy Anomey Larson Taylor & Hmds Authority, London, England M [57] ABSTRACT L51 Filed" May 1970 Relatively opaque and relatively translucent bodies [21] Appl. No.: 35,198 are distinguished from one another by illuminating a limited area of the surface of each body and determining the amount of light emitted from the surface of the [30] Foreign Apphcatlmi P'nomy Data body outside the limited area, the amount of such May 19, 1969 Great Brltam 25529/69 emitted light increasing with increasing transiucency of the body. A coaxial light-transmitting and light- [52] US. Cl. 250/222 R, 350/96 B, 356/210, receiving arrangement is preferred, using a common 250/227 lens, and using adjacent fibre-optic transmitting and [51] Int. Cl. G06m 7/00 receiving surfaces [58] Field of Search 250/227, 222, 223; Th h 350/96 R, 96 B; 356/103, 104 209, 2m 239 e invention as one app lcation n distinguishing between ore fragments WhlCh comprise a translucent 1561 Lariat-2132:1216 i ii ilifififiifrif 7: .g., 1 UNITED STATES PATENTS uraninite) non-uniformly dispersed therein, and can 3,515,273 6/1970 Seaborn .-209/111.7 be applied to physicaiiy separate the fragments 3,068,742 12/1962 Hicks, Jr. et al 250/227 X according to their mineral Content. 3,327,584 6/1967 Kissinger 250/227 X 3,240,106 3/1966 Hicks, Jr. 350/96 R X 20 Claims, 15 Drawing Figures tt 0 1112 2550/222 g a PATENTEDJAN 1 5 I574 SHEET 1 [1F 4 PATENTED 1 51974 3,786,266

SHEET u 0F 4 FIG. 73.

METHODS AND APPARATUS FOR DISTINGUISHING BETWEEN BODIES ACCORDING TO THEIR TRANSLUCENCY BACKGROUND OF THE INVENTION This invention relates to methods and apparatus for distinguishing between bodies according to certain predetermined characteristics, and has one application in the beneficiation of ores, for example uranium ores.

For economic reasons it is desirable to enrich the concentration of the wanted metal in an ore before treating the ore to extract the metal. In some ores the wanted metal is non-uniformly dispersed as an opaque mineral in a matrix of a translucent substance such as quartz or quartzite. For example in one known lowgrade uranium ore (from Lake Elliott, Canada), containing about 0.7 percent of the mineral uraninite, the latter is present as concentrations of small particles (each particle about m in diameter) in a quartz or quartzite matrix. If the ore is reduced to fragments of about 0.25 inch diameter, it is found that about 40 percent of the fragments contain an average of about 400 ,ugm of uraninite per lump, and the remaining 60 percent about ;gm per lump. Beneficiation of such an ore can evidently be effected by a method which separates the percent from the 60 percent.

Various known methods of distinguishing the uranium content of ore fragments include radioactivity measurement, emission spectroscopy, fluorescence measurement etc, but these can be uncertain and inefficient in operation.

The present invention permits use of the observation that, in ore fragments such as those hereinbefore described, those fragments which contain little or none of the wanted mineral are relatively translucent, whereas those which contain most of the mineral are relatively opaque, at least in those parts of the fragments where a mineral concentration is present.

SUMMARY OF THE INVENTION According'to the present invention a method for distinguishing between bodies according to certain predetermined characteristics comprises directing an illuminating beam on to a limited area of the surface of each body and determining the amount of light emitted from the surface of said body outside said limited area. The amount of such emitted light increases with increasing translucency of the material of the body in the illuminated and adjacent areas. 7

The method may comprise forming an image of th surface of the body including said limited area and providing between said image and a photoelectric detector a light-path transversely dimensioned to substantially allow only light from outside said limited area to reach said photoelectric detector.

Said light-path may include occulting means dimensioned to substantially exclude light from said limited area of the image. Alternatively said light-path may include light-conducting means dimensioned to substantially conduct only light from outside said limited area of the image.

The bodies may be fragments of an ore, said frag- Apparatus according to the present invention for distinguishing between bodies according to certain predetermined characteristics comprises means for directing an illuminating beam on to a limited area of the surface of each body and means for determining the amount of light emitted from the surface of said body outside said limited area.

The means for determining the amount of light emitted from the surface of said body outside said limited area may comprise a photoelectric detector, and means for forming an image of the surface of the body. there being provided between said image and the photoelectric detector a light-path which is transversely dimensioned to substantially allow only light from said limited area reaching said photoelectric detector.

Said light-path may include occulting means dimensioned to substantially prevent light from said limited area of the image reaching said detector. Alternatively said light-path may include light-conducting means dimensioned to substantially conduct only light from outside said limited area of the image to the detector.

The optical axes of said illuminating and lightdetermining means may be arranged to converge at the body, or to be substantially coaxial. Where substantially coaxial, they may comprise common imageforming means.

In arrangements in which the axes converge at the body, the occulting means may comprise an opaque bar extending across a photosensitive surface of said photoelectric detector.

The illuminating means may be arranged to provide at least one narrow illuminating beam of light whose long dimension is aligned transversely to a path along which the bodies are arranged to move, whereby said bodies may intercept said beam as they move.

In a substantially coaxial arrangement comprising common image-forming, e.g., lens, means, the illuminating means and light-determining means may com-' prise adjacent areas arranged respectively to transmit light to and to receive emitted light from said body, said transmitting area being connected via a first light-path to a light source and said receiving area being connected via a second light-path to said photoelectric detector. In such an arrangement the transmitting area may be regarded as serving also as an occulting means for preventing light from the limited illuminated area of the body from reaching the photoelectric detector, since in the absence of aberrations and defocussing all this light returns to the transmitting area only. Further occulting means may be provided extending over an area at the boundary between the transmitting and receiving areas to take account of such aberrations and defocussing.

Preferably the common image-forming means is so arranged, in relation to the positions of the bodies and of the transmitting and receiving areas, that said limited illuminated area of the body is a magnified image of the transmitting area.

Preferably a transverse dimension of the transmitting area is smaller than a corresponding dimension of the adjacent receiving area.

Said transmitting and receiving areas may be translucent surfaces and may conveniently be the end-faces of fibre-optic bundle means forming said first and second light-paths. The transmitting area may be a central area surrounded by an annular receiving area. Alternatively the transmitting area may comprise one or morenarrow strips adjacent to wider receiving areas. A plurality of alternate transmitting and receiving strips may be used. Occulting means may be provided extending over an area between the transmitting and receiving endfaces.

The output of the light-determining means may be connected to control means for physically separating relatively translucent from relatively opaque bodies. Various forms of such separating means are known, using, for example, pneumatic jets applied to the falling bodies, or electromagnetically operated push-rods.

DESCRIPTION OF THE DRAWINGS To enable the nature of the present invention to be more readily understood, attention is directed by way of example to the accompanying drawings wherein:

FIGS. la and lb are diagrams illustrating the physical property used in the present invention.

FIGS. 2 and 3 are diagrams illustrating two noncoaxial embodiments of the invention.

FIG. 4 is a diagram illustrating a coaxial embodiment employing fibre-optics.

FIGS. 5, 6 and 7 illustrate forms of fibre-optic transmitting and receiving surfaces.

FIGS. 8, 9, 10, 11 and 12 are diagrams illustrating the optical design of apparatus embodying the present invention.

FIG. 13 illustrates a further form of fibre-optic transmitting and receiving surface.

FIG. 14 is a diagram of an ore-sorting apparatus embodying the present invention.

DESCRIPTION OF EMBODIMENTS Referring to FIG. In, an opaque surface 1 is illuminated by a beam 2 of small cross-section. Light is reflected in all directions as shown by the arrows 3, but only from the illuminated area of the surface.

In FIG. lb the surface 1' consists of translucent crystals 4. The beam 2 passes through the surface, and after multiple internal reflections, part emerges from the surface outside the area illuminated by the beam 2, as shown by the arrows 3'. The effect can be observed if a piece of quartz is illuminated with a narrow laser beam. Surrounding the bright illuminated spot produced by the laser beam is an area which glows with the colour of the light. If the laser beam is directed at a piece of opaque uraninite, this glow is absent.

In FIG. 2 a light beam is directed at a body 5, illuminating a limited area of small diameter at a point A on its surface. Light from the surface of body is imaged at the photocathode 6 of a photomultiplier tube by a lens 7. A bar-like occulting mask 8 is located in front of the photocathode 6. Suitably a laser beam of about 1/10 inch diameter can be used, lens 7 is of 12 inch focal length and the body 5 and photocathode 6 are each about 24 inches from lens 7, and mask 8 is about l/lO inch wide.

If the illuminated area of body 5 is opaque, all the light emitted therefrom will be emitted from the H inch diameter illuminated area and will be intercepted by mask 8, which is in the light-path between image and photocathode. There will therefore be no output from the photomultiplier. If the illuminated area is translucent, light will also be emitted from the body surface outside the illuminated area, and this light will not be intercepted by mask 8. An output will therefore be obtained from the photomultiplier. The effect is also obtained if the translucent material is non-crystalline.

If the illuminated area A were fixed in location relative to lens 7 and photocathode 6, in theory only a central circular mask would be required at A. In practice however, the optical system may have the usual aberrations, and the position of surface A may be uncertain within limits, eg the bodies may be irregularly shaped. as are ore fragments, or their position on a moving belt may not be closely defined. FIG. 2 shows the effect when the illuminated surface is located at points B and C, respectively further and closer than point A. The respective images formed by lens 7 are located at B and C instead of at A. Thus the images formed at photocathode 6 are slightly defocussed, are respectively smaller and larger than the image at A, and are displaced across the photocathode in the plane of the paper. The use ofa bar-shaped mask ensures interception oflight from illuminated areas B and C as shown. Theoretically the bar could taper in width towards the B end, but in practice the reduction in size of the B image makes tapering unjustified.

In FIG. 3 a long, narrow illuminating beam 2' emanating from a slit 9 is used, the long dimension of the beam being transverse to a path 10 along which bodies 5 can move or be moved in a direction normal to the plane of the paper. The long dimension of beam 2' is sufficient to intercept body 5 wherever it may be located across the path. The beam 2' illuminates a limited strip of the upper surface of the body, from which light is reflected via lens 7 to photocathode 6' as before. The occulting bar 8' intercepts light from the illuminated strip but allows any light emitted from the surface of the body on either side of the illuminated strip to reach the photocathode.

Using the apparatus shown in FIG. 2, H4 inch fragments of a sample of Lake Elliott uranium ore of known uraninite content were subjected to the present method. 40 percent of the fragments were distinguished as opaque" and were found to contain percent of the total uranium contained in the complete sample. This result was obtained in a single pass, ie without re-examining the translucent fragments. In practice, the irregular distribution of mineral within each fragment may result in the illumination of a translucent area of a fragment, another area of whose surface is opaque because of the presence of mineral. For this reason it may be desirable to re-examine the apparently translucent fragments after they have been disturbed to reorient their surfaces.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 4 shows a coaxial arrangement which takes advantage of the known light-conducting properties of coated glass or plastic fibres. A fibre-optic bundle comprises portions ll and 12 forming first and second light-paths respectively. Portions l1 and 12 need not be individually coherent. At a common end-face, portion 11 forms a central circular transmitting surface 13 and portion 12 an annular receiving surface 14. Surface 13 is illuminated by a light source 15 and light from surface 14 is transmitted to a photocathode 16. The illuminated surface 13 is imaged on body 5 at 17 by a lens 18, and light from the surface of the body is imaged at the common end-face of the bundle by the same lens. Light reflected from the limited area 17 is imaged on to surface 13, whereas any light emitted from the surface surrounding area 17 is imaged on surface 14. Hence only the latter light is received by photocathode 16. Area 13 can thus be regarded as acting, in effect, as an occulting mask for the light reflected from area 17, although it will be observed that this masking effect is still obtained if area 13 is notionally replaced by an empty hole.

Optical aberrations, and defocussing due to variations in the distance of the surface of body 5 from lens 18, may cause some overspill of reflected light from area 17 on to surface 14. For this reason it may be desirable, in some applications of the invention, to provide an opaque mask 19 which occults an annulus between the transmitting and receiving surfaces, as shown in FIG. 5. Instead of mask 19, other occulting means can be used, such as a fibre-free annular zone or even an empty space; or the fibres in the annular zone can simply not be led to the photocathode. The requirement is that the annular occulting means should not transmit light to the body, and that light received by this occulting means from the body should not be passed to the photoelectric detector.

FIG. 6 shows an alternative form of fibre-optic endface in which the transmitting surfaces 13' are strips alternating with the receiving surfaces 14. The strips can be aligned transversely to a path along which the bodies move, to produce an effect similar to that of FIG. 3.

In FIG. 7 the transmitting surface 13" is of narrow cruciform configuration bounded by receiving surfaces 14".

As in FIG. 5, occulting means (not shown) can be provided at the boundaries between surfaces 13 and 14 (FIG. 6) and 13" and 14" (FIG. 7).

In order to obtain the maximum collection of light from outside the limited illuminated area of the body, it is desirable that the width of the transmitting surface or surfaces 13, etc., should be small relative to the available receiving surface or surfaces 14, etc. The arrangements of FIGS. 6 and 7 are preferable to that of FIG. 5 in this respect, in that they can provide larger illuminating surfaces without necessarily increasing the width thereof.

Means other than fibre-optics can be used to provide adjacent transmitting and receiving areas.

FIGS. 8-12 illustrate how the optical design of a coaxial embodiment of the present invention may be effected, partly by calculation and partly by experiment. It is desirable to direct as much light as possible on to the illuminated area of the ore fragment. This can be done by using a bright light source, such as a xenon arc, a laser, or a tungsten iodide or projector prefocus lamp, and by illuminating the largest feasible area. The conjugate foci are selected to give a depth of field sufficient to cope with the possible variations in position of the fragments. The size and shape of the illuminated area projected on to the fragment is determined mainly by the characteristics of the ore fragments.

FIGS. 8, 9 and 10 show some of the factors involved, related to the characteristics of the ore fragments. s is the area of a fragment 5 illuminated by an incident beam, 1 which is considered of uniform intensity. The flux, 11 scattered from a small area, a, of the fragments surface can be estimated, as a double integral, as fol- The flux d) scattered from a is a function of the incident flux, 1,, dx, d0, and some function of x, F(x).

And the flux scattered as a result of the total incident energy,

This integration can only be completed when the internal scattering characteristics of the ore are known, which can be determined by experiment, and when the shape of the illuminated area has been chosen.

If it is assumed that the ore fragment is isotropic, and that the illuminated area s is a small circular spot ds (FIG. 9), then the scattered flux at radius x from the illuminated area ds is given by The total flux, I scattered by the fragment is I =21rI -dsJ: F(x)- .5x where X the maximum radius which the size and shape of the fragment permit.

It may be assumed that, in all practical ore fragments, F (x) is some inverse function, so that there is some limiting value of x, say x within which most, say percent, of the scattered radiation lies. This parameter, x is used to define the optimum dimensions of the illuminating area. In FIG. 10, the illuminated area ds is shown as a narrow strip, and an area a distant x from the strip is considered. As the strip ds is widened, indicated by moving the lower edge of the strip away from a (see interrupted lines), the proportion of light scattered from a which originated at the lower edge of the strip ds falls until it is insignificant, i.e., when the strip edge is x, distant from a. Evidently the maximum useful width of the illuminated strip ds is X It is also apparent that a strip-shaped illuminated area is preferable to a circular shape, on the basis of the above analysis, because it allows a larger area on which light can be thrown. This is significant if the light source is basically thermal, e.g., a tungsten iodide lamp, but is not necessarily significant if a laser is used because, with a highly collimated source such as a laser, practically all the energy from the laser can be used, and the shape of the illuminated area merely determines its distribution.

x is therefore an important parameter, which can be determined experimentally from actual ore fragments.

Considering now the geometrical optics, the value of x, is an important piece of data. Amongst other things it helps to determine factors such as permitted magnification, illuminating aperture dimensions, and size of occulting mask.

For example, referring to FIGS. 11 and 12, in a coaxial arrangement such as that illustrated by FIG. 4, the size of the illuminated area h formed by projecting an illuminating aperture h on to the fragment 5, is

h mh where m is the magnification, L, /L

If h x, as previously described, then FIG. 11 shows the illuminating system. The returned light is pre-focussed over the illuminating aperture, as shown in FIG. 12. In FIG. 12, it is assumed that the ore fragment has been displaced axially through a distance 53y, from the position of best focus. In consequence, an image h is formed at some distance L from the lens, and the light patch falling on the occulting mask will be larger than h,,. This image is that of the light reflected directly from the surface of the fragment. The light internally scattered before re-appearing at the surface of the fragment is imaged outside image h as previously described.

If Sli is the increment in size of the light patch falling on the occulting mask, then it can be shown that m 2 D )12/ )f1 where D is the diameter andf is the focal length of lens 18. The minimum effective size of occulting mask is thus h h, Sh

Taking account of the fragments characteristics,

since (0) the occulting mask size,

i D5111: m+ )f1) (9) It is apparent from the above that to obtain the largest value for y", ie the largest depth of field, m should be large.

A further consideration is the illumination efficiency of the system. This is determined firstly by the numerical aperture of the illuminating means, i.e., D/L and secondly the numerical aperture of the light-collecting means, i.e., D/L

Combining these two, and assuming a Lambertian emitter for the illuminating source, the energy returned to the occulting mask is proportional to where N D/f This shows that for a given aperture D and first conjugate L it pays to make m as small as possible. This is in contradiction to the depth-of-field requirement, so that a compromise has to be reached between the need to achieve a given depth of field and a good light efficiency.

achieving a desired result, and it will be noted that the result depends on the experimentally determined ore characteristics.

Some other factors which may also affect the design will now be considered. One of these is the size of the fragments. It may be that, for non-optical reasons, the fragments are too small to allow the full optimum illuminated area size, x,, to be used. The main effect of this will be to restrict the size of illuminating aperture and hence the total signal for any given light source except a laser used in a collimated condition.

Furthermore, it may be that the fragments are so large that they can accommodate several illuminated areas, in which case a multiple aperture system, e.g., of .parallel strips as in FIG. 6, can be used.

Alternatively, the fragments may be such that they have only small areas of opacity which it may be important to identify. In such cases, it will be necessary to use a relatively small illuminated area and to subject each fragment to several examinations.

Additionally, the occulting means may have to be larger than purely geometrical considerations would suggest, because of, e.g., scattering at the fragments surface from those areas directly illuminated to adjacent crystal facets, and also because of aberrations within the projection optics. These factors also may require an occulting area to be provided between the adjacent illuminating (ie transmitting) and receiving surfaces.

It will now be shown numerically how the relationships established above affect the design of a system.

The tolerated depth of field, 8y is determined mainly by two factors, viz. positional variations of the fragments, and size variations. Rewriting equation (7) the variation of occulting mask size with depth of field is found. From this expression the optical proportions of the system can be chosen, e.g.: Table 1 gives values of 8h, /8y for values of m and N:

The dividing line shows the permitted relationships of m and N to achieve a depth-of-field/aperture-change ratio of 100, viz. those relationships which give values on the right hand side of the line. The ratio of 100 is chosen by way of example and is not critical.

The results ofTable l are now compared with the illumination efficiencies, as given by equation (10). Table 2 shows the value of m'- N /(m+1) for the The above analysis indicates the design procedure for ame values of m and N as in Table 1.

TABLE 2 1. 03 10 1 20 10 3 osxio- 1 03x10- 1.20 10 302900- Comparison of Tables 1 and 2 shows that to obtain a good depth of field together with the maximum light signal the optical system should be designed to use a large numerical aperture lens with a moderate magnification.

The foregoing analysis will now be applied to the design of a practical coaxial system using fibre optics by way of example. Let it be assumed that the ore is of such a nature that x, 0.1 inch (found experimentally) and that the average diameters of the fragments lie between 0.3 and 0.5 inch. Evidently an illuminated area of 0.1 inch diameter or width (depending on whether the illuminating aperture is circular or strip-like) can be used, since the surface from which scattered light of useful intensity is returned (0.3 inch edge-toedge) will be within the bounds of the fragment. Allowing for an increase in returned image size of not more than 0.01 inch for a 0.25 inch change in the position of the fragment, i.e.,

6 Il /Sy 0.04,

it is evident from Table 1 that with a magnification of four the largest numerical aperture is 0.5 f/2), in which case the illuminating aperture h is.0.025 inch in diameter or width (dimension p in FIGS. and 6). The occulting mask 19 requires to be at least 0.035 inch in outer diameter or edge-to-edge width (dimension q'in FIG. 5). The full receiving aperture requires to be at least 0.3/4 inch in outer diameter or edge-to-edge width, i.e., 0.075 inch (dimension r in FIGS. 5 and 6).

Taking a lens 18 of f/2 aperture and 1.5 inch focal length, the long conjugate L is 7.5 inches and the short conjugate L is 1.875 inches. From Table 2 the illumination coefficient is 1.60 X 10 which is some 10 times greater than a system using m l and having the same depth of field.

FIG. 13 shows a fibre-optic end-face designed in accordance with the above calculations, comprising a transmitting surface 113 and receiving surfaces 114. The latter are separated from the transmitting surface by occulting strips 119 which in this embodiment are pieces of aluminium foil aligned edge-on to the endface. The fibres and foils are embedded in a block 20 of an epoxy resin in a known manner. The dimensions of this end-face are r 0.135 inch p 0.025 inch q 0.035 inch t= 0.10 inch i.e., the surface 113 is 0.025 inch wide, the foils 1119 each 0.005 inch thick and the surfaces 114 each 0.050 inch wide.

The fibres from surface 113 are led to a light-source 15 (see FIG. 4) suitably comprising a quartz-iodine lamp, and the fibres from the two surfaces 114 are led to a detector suitably comprising a silicon photo-diode as an alternative to the photomultiplier tube described hereinbefore. The lens 18 of 172 aperture has a focal length of 1.5 inches and the lens conjugates are as calculated above.

Apparatus according to the present invention can be connected to control known means for physically separating the opaque from the translucent bodies. Apparatus for ejecting selected bodies from -a falling stream thereof using pneumatic jets is shown, for example, in

U. S. Pat. Nos. 3,011,634, 3,097,744 and 3,075,641. FIG. 14 shows an example of the present apparatus so connected. Ore fragments from a bin 21 are fed to a moving belt 22 and fall from the end thereof as a stream 23. In falling, they pass before a housing 24 which contains the lens 18 and fibre-optic endface of the present apparatus. The end-face is of the 'kind shown in FIG. 13 and has the long dimension (1) of its transmitting surface aligned horizontally. The fibres 11 from the transmitting surface are led to light-source consisting of a quartz-iodine lamp and the fibres from the receiving surfaces to a detector 116 comprising a silicon photo-diode. The output from the latter is fed to a control unit 25 which is connected to control a pneumatic valve 26. The latter is connected between a compressed air supply 27 and a nozzle 28.

Below the nozzle 28 is a divider 29 located so that ore fragments undeflected by a jet of air from the nozzle fall to the right hand side thereof, but fragments deflected by a jet of air fall to the left hand side thereof. Bins 30 and 31 receive the fragments falling to the respective sides.

As the ore fragments pass before housing 24 they are illuminated by the present apparatus as hereinbefore described. If the resulting output from the photo-diode I6 exceeds a preset threshold, this indicates that the fragment momentarily opposite-housing 24 is primarily translucent (i.e., contains little or no uraninite), and, after a delay corresponding to the time of fall between housing 24 and nozzle 28, control unit 25 causes nozzle 28 to emit a puff of air which deflects the fragment in question to the left hand side of the divider. Thus the uranium-containing (primarily opaque) fragments are not deflected and fall into bin 30, whereas the fragments containing little or no uranium (primarily translucent) are deflected and fall into bin 31. The system can alternatively be arranged to deflect the primarily opaque (uranium-containing) fragments from the stream.

Apparatus operating in the manner shown in FIG. 14 has been applied to sorting fragments of Lake Elliott ore in the range 0.2 to 0.4 inch diameter, with results comparable to those given for the apparatus of FIG. 2.

Although described with reference to its use for distinguishing uranium ore fragments, the invention is not limited to such ores but can be used for other minerals having variable translucency. It can also be applied to non-mineral bodies having appropriate translucency characteristics.

We claim:

1. A method for distinguishing between bodies according to their translucency characteristics, said bodies comprising fragments of ore and said fragments containing different amounts of an opaque mineral in a translucent matrix, said method comprising directing,

an illuminating beam from a source located at a dis-. tance from the bodies to illuminate a limited sharply defined area of the surface of a body and determining the amount of light emitted from the surface outside of but bordering said limited area, the second-mentioned step including imaging said limited sharply defined area and said bordering surface substantially at a plane including areas which are respectively in lighttransmitting communication and non-communication with a light-sensitive detecting means, said limited sharply-defined area being imaged at said plane within the non-communicative area and at least part of said bordering surface being imaged at said plane within the communicative area.

2. A method as claimed in claim 1 further comprising using transversely dimensioned occulting means located at said plane to substantially exclude light from said limited area of the image.

3. A method as claimed in claim 1 wherein the opaque mineral is a uranium mineral such as uraninite and the matrix is substantially quartz or quartzite.

4. A method for distinguishing between bodies according to their translucency characteristics, said bodies comprising fragments of ore and said fragments containing different amounts of an opaque mineral in a translucent matrix, said method comprising directing an illuminating beam from a source located at a distance from the bodies to illuminate a limited sharply defined area of the surface of a body and determining substantially only the amount of light emitted from the surface outside of but bordering said limited area while rejecting the light reflected back from said limited area, the second-mentioned step including imaging said limited sharply-defined area and said bordering surface substantially at a plane including areas which are respectively in light-transmitting communication and non-communication with a light sensitive detecting means, said limited sharply defined area being imaged at said plane within the non-communicative area and at least part of said bordering surface being imaged at said plane within the communicative area.

5. A method as claimed in claim 4 further comprising light-conducting means connected between said plane and said detecting means, said light-conducting means having a cross-section at said plane dimensioned to receive substantially only light from said image of said bordering surface and to exclude light from said image of said limited defined area.

6. Apparatus for distinguishing between bodies according to their translucency characteristics comprising means located at a distance from the bodies for directing an illuminating beam to illuminate a limited sharply defined area of the surface of a said body, and means for determining the amount of light emitted from the surface outside but bordering said limited area, the second-mentioned means including lightsensitive detecting means and optical means for imaging said limited sharply defined area and said bordering surface substantially at a plane, said plane including areas which are respectively in light-transmitting communication and non-communication with said lightsensitive detecting means, and said optical means imaging said limited defined area at said plane within the non-communicative area and imaging at least part of said bordering surface at said plane within the communicative area, said apparatus further comprising occulting means located at said plane and transversely dimensioned so to prevent light from said limited area of said apparatus from reaching said detecting means.

7. Apparatus as claimed in claim 6 wherein the optical axes of said illuminating and light-sensitive detecting means are arranged to converge at the position of the body.

8. Apparatus as claimed in claim 6 wherein the optical axes of said illuminating and light-sensitive detecting means are coaxial and comprise common imageforming means.

9. Apparatus for physically separating relatively translucent from relatively opaque bodies comprising apparatus as claimed in claim 6 wherein the output of said light-determining means is connected to control physical separation means.

10. Apparatus as claimed in claim 8 wherein the illuminating means and light-determining means comprises adjacent areas arranged respectively to transmit light to and to receive emitted light from said body, said transmitting area being connected via a first light-path to a light source and said receiving area being connected via a second light-path to said photoelectric detector.

11. Apparatus as claimed in claim 10 wherein the common image-forming means is arranged, in relation to the positions of the bodies and of the transmitting and receiving, areas, so that said limited illuminated area of the body is a magnified image of the transmitting area.

12. Apparatus as claimed in claim 10 wherein a transverse dimension of the transmitting area is smaller than a corresponding dimension of the adjacent receiving area.

13. Apparatus as claimed in claim 10 wherein said transmitting and receiving areas are translucent surfaces.

14. Apparatus as claimed in claim 13 wherein said translucent surfaces are the end-faces of fibre-optic bundle means forming said first and second light-paths.

15. Apparatus as claimed in claim 14 wherein the transmitting area is a central area surrounded by an annular receiving area.

16. Apparatus as claimed in claim 14 wherein the transmitting area comprises at least one narrow strip located between two wider receiving areas.

17. Apparatus as claimed in claim 16 wherein occulting means are provided between the bundles forming the transmitting and receiving areas respectively adjacent said end-faces.

18. Apparatus as claimed in claim 7 wherein the illuminating means is arranged to provide at least one narrow illuminating beam of light whose long dimension is aligned transversely to a path along which the bodies are arranged to move, whereby said bodies may intercept said beam as they move.

19. Apparatus for distinguishing between translucent bodies according to their translucency characteristics comprising means located at a distance from the bodies for directing an illuminating beam to illuminate a limited sharply defined area of a said body, and means for determining substantially only the amount of light emitted from the surface outside but bordering said limited area while rejecting the light reflected back from said defined area itself, said second-mentioned means including light-sensitive detecting means and optical means for imaging said limited defined area and said bordering surface substantially at a plane, said plane including areas which are respectively in lighttransmitting communication and non-communication with said light-sensitive detecting means, said optical means imaging said limited defined area at said plane within the non-communicative area and imaging at least part of said bordering surface at said plane within the communicative area.

20. Apparatus as claimed in claim 19 further comprising light-conducting means connected between said plane and said detecting means, said said lightconducting means having a cross-section at said plane dimensioned to receive substantially only light from said image of said bordering surface and to exclude light from said image of said limited defined area. 

1. A method for distinguishing between bodies according to their translucency characteristics, said bodies comprising fragments of ore and said fragments containing different amounts of an opaque mineral in a translucent matrix, said method comprising directing an illuminating beam from a source located at a distance from the bodies to illuminate a limited sharply-defined area of the surface of a body and determining the amount of light emitted from the surface outside of but bordering said limited area, the second-mentioned step including imaging said limited sharplydefined area and said bordering surface substantially at a plane including areas which are respectively in light-transmitting communication and non-communication with a light-sensitive detecting means, said limited sharply-defined area being imaged at said plane within the non-communicative area and at least part of said bordering surface being imaged at said plane within the communicative area.
 2. A method as claimed in claim 1 further comprising using transversely dimensioned occulting means located at said plane to substantially exclude light from said limited area of the image.
 3. A method as claimed in claim 1 wherein the opaque mineral is a uranium mineral such as uraninite and the matrix is substantially quartz or quartzite.
 4. A method for distinguishing between bodies according to their translucency characteristics, said bodies comprising fragments of ore and said fragments containing different amounts of an opaque mineral in a translucent matrix, said method comprising directing an illuminating beam from a source located at a distance from the bodies to illuminate a limited sharply-defined area of the surface of a body and determining substantially only the amount of light emitted from the surface outside of but bordering said limited area while rejecting the light reflected back from said limited area, the second-mentioned step including imaging said limited sharply-defined area and said bordering surface substantially at a plane including areas which are respectively in light-transmitting communication and non-communication with a light sensitive detecting means, said limited sharply-defined area being imaged at said plane within the non-communicative area and at least part of said bordering surface being imaged at said plane within the communicative area.
 5. A method as claimed in claim 4 further comprising light-conducting means connected between said plane and said detecting means, said light-conducting means having a cross-section at said plane dimensioned to receive substantially only light from said image of said bordering surface and to exclude light from said image of said limited defined area.
 6. Apparatus for distinguisHing between bodies according to their translucency characteristics comprising means located at a distance from the bodies for directing an illuminating beam to illuminate a limited sharply-defined area of the surface of a said body, and means for determining the amount of light emitted from the surface outside but bordering said limited area, the second-mentioned means including light-sensitive detecting means and optical means for imaging said limited sharply-defined area and said bordering surface substantially at a plane, said plane including areas which are respectively in light-transmitting communication and non-communication with said light-sensitive detecting means, and said optical means imaging said limited defined area at said plane within the non-communicative area and imaging at least part of said bordering surface at said plane within the communicative area, said apparatus further comprising occulting means located at said plane and transversely dimensioned so to prevent light from said limited area of said apparatus from reaching said detecting means.
 7. Apparatus as claimed in claim 6 wherein the optical axes of said illuminating and light-sensitive detecting means are arranged to converge at the position of the body.
 8. Apparatus as claimed in claim 6 wherein the optical axes of said illuminating and light-sensitive detecting means are coaxial and comprise common image-forming means.
 9. Apparatus for physically separating relatively translucent from relatively opaque bodies comprising apparatus as claimed in claim 6 wherein the output of said light-determining means is connected to control physical separation means.
 10. Apparatus as claimed in claim 8 wherein the illuminating means and light-determining means comprises adjacent areas arranged respectively to transmit light to and to receive emitted light from said body, said transmitting area being connected via a first light-path to a light source and said receiving area being connected via a second light-path to said photoelectric detector.
 11. Apparatus as claimed in claim 10 wherein the common image-forming means is arranged, in relation to the positions of the bodies and of the transmitting and receiving areas, so that said limited illuminated area of the body is a magnified image of the transmitting area.
 12. Apparatus as claimed in claim 10 wherein a transverse dimension of the transmitting area is smaller than a corresponding dimension of the adjacent receiving area.
 13. Apparatus as claimed in claim 10 wherein said transmitting and receiving areas are translucent surfaces.
 14. Apparatus as claimed in claim 13 wherein said translucent surfaces are the end-faces of fibre-optic bundle means forming said first and second light-paths.
 15. Apparatus as claimed in claim 14 wherein the transmitting area is a central area surrounded by an annular receiving area.
 16. Apparatus as claimed in claim 14 wherein the transmitting area comprises at least one narrow strip located between two wider receiving areas.
 17. Apparatus as claimed in claim 16 wherein occulting means are provided between the bundles forming the transmitting and receiving areas respectively adjacent said end-faces.
 18. Apparatus as claimed in claim 7 wherein the illuminating means is arranged to provide at least one narrow illuminating beam of light whose long dimension is aligned transversely to a path along which the bodies are arranged to move, whereby said bodies may intercept said beam as they move.
 19. Apparatus for distinguishing between translucent bodies according to their translucency characteristics comprising means located at a distance from the bodies for directing an illuminating beam to illuminate a limited sharply defined area of a said body, and means for determining substantially only the amount of light emitted from the surface outside but bordering said limited area while rejecting the light reflected back from said defined area itself, said second-mentioned means including light-sensitive detecting means and optical means for imaging said limited defined area and said bordering surface substantially at a plane, said plane including areas which are respectively in light-transmitting communication and non-communication with said light-sensitive detecting means, said optical means imaging said limited defined area at said plane within the non-communicative area and imaging at least part of said bordering surface at said plane within the communicative area.
 20. Apparatus as claimed in claim 19 further comprising light-conducting means connected between said plane and said detecting means, said said light-conducting means having a cross-section at said plane dimensioned to receive substantially only light from said image of said bordering surface and to exclude light from said image of said limited defined area. 